The present invention is drawn to somatostatin-dopamine chimeric analogs and methods relating to their therapeutic use.
Dopamine is a catecholamine neurotransmitter that has been implicated in the pathogenesis of both Parkinson disease and schizophrenia. Dopamine and related molecules have been shown to inhibit the growth of several types of malignant tumors in mice, and this activity has been variously attributed to inhibition of tumor-cell proliferation, stimulation of tumor immunity or as well as effects on melanin metabolism in malignant melanomas. Recent studies demonstrated the presence of D2 dopamine receptors on endothelial cells. Dopamine has recently been reported to strongly and selectively inhibit at non-toxic levels the vascular permeabilizing and angiogenic activities of VPF/VEGF.
Somatostatin (SS), a tetradecapeptide has been shown to have potent inhibitory effects on various secretory processes in tissues such as pituitary, pancreas and gastrointestinal tract. SS also acts as a neuromodulator in the central nervous system. These biological effects of SS, all inhibitory in nature, are elicited through a series of G protein coupled receptors, of which five different subtypes have been characterized (SSTR-1-SSTR-5). These five subtypes have similar affinities for endogenous SS ligands, but have differing distributions in various tissues. Somatostatin binds to the five distinct receptor (SSTR) subtypes with relatively high and equal affinity for each subtype.
There is evidence that SS regulates cell proliferation by arresting cell growth via SSTR-1, -2, -3, -4, and -5 subtypes, and/or by inducing apoptosis via SSTR-3 subtype. SS and various analogues have been shown to inhibit normal and neoplastic cell proliferation in vitro and in vivo via specific SS receptors (SSTR's) and possibly different postreceptor actions. In addition, there is evidence that distinct SSTR subtypes are expressed in normal and neoplastic human tissues, conferring different tissue affinities for various SS analogues and variable clinical response to their therapeutic effects.
Binding to different types of somatostatin receptor subtypes is associated with the treatment of various conditions and/or diseases. For example, the inhibition of growth hormone has been attributed to the somatostatin type-2 receptor (“SSTR-2”), while the inhibition of insulin has been attributed to the somatostatin type-5 receptor (“SSTR-5”). Activation of types 2 and 5 have been associated with growth hormone suppression and more particularly growth hormone secreting adenomas (acromegaly) and thyroid stimulating hormone (TSH) secreting adenomas. Activation of type 5 but not type 2 receptor has been associated with treating prolactin secreting adenomas. Other indications associated with activation of the somatostatin receptor subtypes include inhibition of insulin and/or glucagon for treating diabetes mellitus, angiopathy, proliferative retinopathy, dawn phenomenon, and nephropathy; inhibition of gastric acid secretion for treating peptic ulcers, enterocutaneous and pancreaticocutaneous fistula, irritable bowel syndrome, Dumping syndrome, watery diarrhea syndrome, AIDS related diarrhea, chemotherapy-induced diarrhea, acute or chronic pancreatitis and gastrointestinal hormone secreting tumors; treatment of cancer such as hepatoma; inhibition of angiogenesis; treatment of inflammatory disorders such as arthritis; retinopathy; chronic allograft rejection; angioplasty; preventing graft vessel and gastrointestinal bleeding. Preferably, a somatostatin analog is selective for the specific somatostatin receptor subtype or subtypes responsible for the desired biological response to reducing interaction with other receptor subtypes which could lead to undesirable side effects or loss of efficacy.
Somatostatin and its receptors (SSTR-1 to SSTR-5) are expressed in normal human parafollicular C cells and medullary thyroid carcinoma (MTC). MTC is a tumor originating from thyroid parafollicular C cells that produces calcitonin (CT), somatostatin, and several other peptides. It was recently demonstrated that SS and SSTR's are expressed in human MTC, and SS and SS analogues were shown to induce a decrease in plasma CT levels and provide symptomatic improvement in MTC patients. Another recent study has shown that SS and SS analogs, in particular, SSTR-1 and SSTR-2, can inhibit the proliferation of tumor cells, suggesting that specific SSTR subtypes can function in MTC cell growth regulation. The development and characterization of SSTR subtype analogues that selectively effect MTC cell growth is useful for clinical and therapeutic applications.